Wheelchairs are presently used by people who have temporarily or permanently lost the ability to walk. In general, there are two types of wheelchairs, namely, those which are manually powered and those which have some form of power assist drive.
With respect to the manually driven wheelchairs, such wheelchairs typically have relatively small swivelling front wheels and large rubber rimmed rear wheels. Gripping rails may be provided on the rear wheels so that persons may propel themselves along by pulling down on the wheels or the gripping rims. A problem arises however in respect of low arm-strength wheelchair riders. In some cases the persons needing wheelchairs will not have sufficient arm strength to propel the wheelchairs along manually. This is especially true in respect of upramps, downramps and curbs which all require additional arm strength to be negotiated.
One solution is to provide motorized wheelchairs which need only be steered. However, the cost of fully motorizing a wheelchair is prohibitive and thus a fully motorized wheelchair is an appropriate solution in only a small number of cases. What is preferred in most cases is some other lower cost solution. Examples of such low cost solutions exist, and include portable units which may be attached to the frame of manual wheelchairs to provide drive to the wheelchairs when needed, but when not needed can be raised out of the way.
One example of such a device is contained in U.S. Pat. No. 4,759,418 entitled WHEELCHAIR DRIVE to Goldenfeld et al. This patent discloses an electric motor attached to a drive wheel and mounted on a carrier. The device may be pivoted about an attachment to the frame between a raised and lowered position. The pivoting is accomplished by means of a cable, an end of which is conveniently placed within reach of the wheelchair passenger. When the cable tension is released, the drive wheel and motor pivot down under their own weight to a ground engaging position. When the drive wheel is pushing against resistance, a force component keeps the drive wheel engaged with the ground to propel the wheelchair. In the absence of such resistance, there is nothing keeping ground engaging contact.
This device may not be used for braking when going down a downramp. This is unsatisfactory and unsafe.
Another solution is proposed in U.S. Pat. No. 3,905,437 to Kaiho et al. entitled ELECTRICALLY DRIVEABLE WHEELCHAIR. Kaiho discloses a drive wheel attached to a motor housing which is moved between a raised and lowered position by a jack. A single motor is used, and upon being energized, both the drive wheel and the jack begin turning. The jack lowers the spinning drive wheel into contact with the ground. Thus, when the motor is running, the jack and the drive wheel are turning. This results in a very dangerous situation since the motor must be reversed to raise the device, hence the drive wheels will rotate backwardly causing the wheelchair to briefly back up. This is dangerous since if the device was used to reach the top of a ramp it potentially could pull the wheelchair occupant back down the ramp while it is being retracted.
Kaiho recognizes the problem by providing a free wheel between the electric motor and the drive wheel so that the motor only communicates forward drive to the drive wheel. However, this means that the device cannot be reversed to back up the wheelchair. Also, Kaiho's device cannot be used for downramps since the wheel is free to pivot rearwardly.
What is desired therefore is a power assist device to overcome the problems of these other devices. Preferably it should be mounted to the frame of an ordinary manual wheelchair and should be relatively inexpensive. Further, it should be movable from a raised position to a lowered position and in the lowered position should be operable on up and down ramps. It should be capable of working forwardly and may be adapted to work in reverse.